This invention relates to secondary electrochemical, power-producing cells that are capable of providing high current densities, high capacity densities and long cycle lives. In more detail, the invention relates to a high-temperature cell that employs molten lithium metal as an anode reactant and a chalcogen such as sulfur or a metallic sulfide as the cathode reactant. The cell includes an improved anode (negative electrode) that exhibits good retention of molten lithium metal during cycling.
A substantial amount of work has been done in the development of electrochemical cells for applications requiring a high specific energy (watt hours per kilogram; W-hr/kg) and a long cycle life (number of charge/discharge cycles before failure). One such application is a battery power source for an electrically powered vehicle. For such an application, the high specific energy requirement must be achieved without sacrificing peak specific power (watts per kilogram; W/kg). Depending upon the design and materials of an electric passenger vehicle, a range approaching 200 miles and an acceptable acceleration capability would require a battery having a specific energy of at least 200 W-hr/kg and a specific power of at least 200 W/kg.
One class of cells that show promise for development of high-specific-energy, high-specific-power secondary batteries are those that operate at elevated temperatures with molten electrolyte. These cells employ lithium or sodium anodes and cathode materials of group VI or VII of the periodic table, the chalcogens and halogens. Of these cells, the lithium/sulfur cell having electrolyte containing lithium ions has proven to be very promising. Examples of such cells are given in U.S. Pat. Nos. 3,827,910 to Cairns et al., entitled "Homogeneous Cathodoe Mixtures for Secondary Electrochemical Power Producing Cells", Aug. 6, 1974, and 3,716,409 to Cairns et al., entitled "Cathodes for Secondary Electrochemical Power Producing Cells", Feb. 13, 1973; both assigned to the assignee of the present application.
In some cells the positive electrode employs a metallic sulfide as opposed to elemental sulfur as the reactant. Such cells are illustrated in U.S. patent application Ser. No. 434,459, filed Jan. 18, 1974, entitled "Cathode for a Secondary Electrochemical Cell", to Gay et al. In addition, various types of anode materials such as lithium-aluminum alloy and molten mixtures of lithium and other metallic additives have been investigated in attempts to achieve stability of the anode with cycling.
In order to maximize both specific energy and specific power of a given electrochemical cell, elemental lithium is a preferred choice for an anode reactant material due to its extremely low electronegativity and low equivalent weight. However, substantial problems have arisen in providing an anode structure or substrate capable of retaining lithium metal charge with cycling. One material that has proven reasonably successful as an anode substrate and current collector is a porous stainless steel matrix having about 80 to 90% void volume and about 65 to 70 microns average pore size. This material can be composed of a random array of stainless steel fibers compressed in a random fashion and sintered at high temperatures, thereby creating a porous metal. Stainless steel Feltmetal (a trademark) is a commonly available material that has been employed with some success as a porous substrate for retaining molten lithium metal in electrochemical cells.
A continuing effort is being carried out to find improved anode substrate materials. High current densities (mA/cm.sup.2) are necessary to achieve high power densities. At current densities of about 200 mA/cm.sup.2 and above, which probably will be required for acceptable operation of an electric vehicle for personal and family transportation, the problem of lithium loss from the anode is intensified. In the continuing search for improved materials and components that will meet these problems, it is also desirable that less complex and more inexpensive materials be found to provide a power source for an electric vehicle that is economically attractive.